1. Field of the Invention
The present invention relates to a deinking method for deinking wastepapers such as newspapers, leaflets and magazines to reclaim them. More particularly, the present invention relates to a deinking process according to which bulkier pulp sheets can be obtained.
2. Prior Arts
Wastepapers are reclaimed by stripping ink from the wastepapers by a deinking treatment to prepare reclaimed pulp and manufacturing reclaimed paper from the reclaimed pulp. The conventional deinking methods generally comprises a step of stripping ink from wastepapers and a step of discharging the stripped ink.
More specifically, the deinking method comprises as main steps:
(1) a step of pulping (disintegrating) wastepapers, PA1 (2) a step of aging, i.e., leaving the disintegrated paper as it is, PA1 (3) a step of flotation, and PA1 (4) a step of washing.
That is, in deinking treatment, ink bound to the fibers of wastepapers is physically and chemically (or biochemically) stripped therefrom to thereby separate the ink from the fibers. Thus, reclaimed pulp is obtained.
The foregoing ink stripping in the conventional deinking treatment is generally carried out at a high pH (pH: 10 to 9), followed by the removal of the ink under the same conditions in the flotation step.
In recent years, the properties of wastepapers which are used as the starting material have been varied because of an increase in a demand for a beautiful and stable printing and a progress in printing techniques. Besides, the degree of bond of ink to paper is widely varied from a mild to strong one. Under these circumstances, it is demanded to improve the quality and the recovery (i.e., productivity) of deinked pulp. In order to attain these purposes, various proposals are offered from various field including deinking processes, deinking agents and apparatuses used for deinking.
However, most of the techniques heretofore proposed are directed to an improvement in the whiteness of deinked pulp. Thus, it cannot be said that sufficient investigations have been made on the strength and bulk density of pulp sheets. Although the pulping step and the flotation step in the conventional deinking method are generally carried out at a pH value on the alkaline side such as a pH of 10 to 9 as described hereinbefore, it cannot be said that the paper strength and bulk density of the resulting pulp sheets are satisfactory. Although there are known some deinking methods wherein the pulping step and the flotation step are carried out at a pH of at most 9 (JP-A 54-23705, JP-A 59-53532 and U.S. Pat. No. 4,043,908), these methods are not necessarily effective as measures for improving the strength and bulk density of pulp sheets.